[Bug 221674] Installer can't back up when it finds MBR

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221674

--- Comment #5 from MMacD <scratch65535 at att.net> ---
(In reply to Nathan Whitehorn from comment #4)

"It's one installer, the issue is that it has no idea if the machine has CSM or
not. You can't know that unless you booted CSM. "

I apologise if this is a dopey question, but I've never had occasion to look at
specific load-and-go problems.  Despite being comfortable with assembly
languages, my interests have always been up at the human end (not surprising,
perhaps, in someone trained as a psychologist).

My mental model of what's going on is that the bios (or equivalent) spins up
the cd drive (or equivalent), loads the primary boot, and jumps to its
well-known start address, carrying on from there. I.e., it's at least a
somewhat agnostic operation at that point, capable of going down either of two
paths.

Why wouldn't that be a good place to add somebody who could take a diagnostic
look at the disc and present something like my menu suggestion?  Such code
would have the same access as any later code for doing low-level reads in aid
of figuring out what's on the disc.

Again: apologies if this is a dopey question, and I'd be happy in that case to
be clued up if you feel you can take the time.

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